1. Field of the Invention
The present invention relates to the enhancement of position estimating systems with the ability to pinpoint the physical location of a target communication device using direct ranging techniques between search communication devices and the target communication device.
2. Description of the Related Art
The capability to rapidly and accurately estimate the physical position of a mobile communication device would be of great benefit in a variety of applications. In a military context, it is desirable to know the location of military personnel and/or equipment during coordination of field operations and rescue missions, including scenarios where signals of conventional location-determining systems, such as global location system (GPS) signals, may not be available (e.g., within a building). More generally, appropriately equipped mobile communication devices can be used to track the location of personnel and resources located both indoors or outdoors, including but not limited to: police engaged in tactical operations; firefighters located near or within a burning building; medical personnel and equipment in a medical facility or en route to an emergency scene, including doctors, nurses, paramedics and ambulances; and personnel involved in search and rescue operations. An integrated location communication device would also allow high-value items to be tracked and located, including such items as personal computers, laptop computers, portable electronic devices, luggage, briefcases, valuable inventory, and stolen automobiles. In urban environments, where conventional location determining systems have more difficulty operating, it would be desirable to reliably track fleets of commercial or industrial vehicles, including trucks, buses and rental vehicles. Tracking of people carrying a mobile communication device is also desirable in a number of contexts, including, but not limited to: children in a crowded environment such as a mall, amusement park or tourist attraction; location of personnel within a building; and location of prisoners in a detention facility.
The capability to estimate the position of a mobile communication device also has application in locating the position of cellular telephones. Unlike conventional land-based/wire-connected telephones, the location of conventional cellular telephones cannot be automatically determined by emergency response systems (e.g., the 911 system in the United States) when an emergency call is placed. Thus, assistance cannot be provided if the caller is unable to speak to communicate his or her location (e.g., when the caller is unconscious, choking or detained against will). The capability to determine the location of cellular telephones can be used to pinpoint the location from which an emergency call has been made. Such information can also be used to assist in cell network management (for example, by factoring each mobile communication device's location into message routing algorithms).
The aforementioned needs are addressed by two co-pending U.S. patent applications: Ser. No. 09/365,702, entitled “Method and Apparatus for Determining the Location of a Mobile Communication Device Using Low Accuracy Clocks,” filed Aug. 2, 1999; and Ser. No. 09/777,625, entitled “Method and Apparatus for Determining the Location of a Mobile Communication Device,” filed Feb. 6, 2001. Both applications are hereby incorporated by reference in their entirety and are hereinafter referred to, jointly, as “patent applications '702 and '625.”
Patent applications '702 and '625 disclose a spread spectrum based technology that allows individuals and/or equipment to be tracked to very close tolerances, even within heavily obstructed environments, such as within “urban canyons” or within modem buildings. The system disclosed employs two-way transmission of spread spectrum ranging signals between mobile communication devices having relatively low accuracy clocks, to estimate rapidly and accurately the location of the mobile communication devices in the presence of severe multipath interference. The technology is small enough to be embedded in radios and/or cell phones, and allows ranging/tracking capabilities to be provided without disrupting other voice or data communications supported by the same device. Prototype devices have demonstrated performance capabilities under a variety of obstructed, multipath conditions.
The disclosed location estimation system accurately and reliably estimates the three-dimensional location of a handheld, portable or vehicle-mounted, spread spectrum communication device within milliseconds without interruption of voice or data communications. Using spread spectrum waveforms and processing techniques, the disclosed system is capable of estimating location to an accuracy of less than one meter in a severe multipath environment. More particularly, these systems employ a two-way, round-trip ranging signal scheme in which the time of arrival of the ranging signals is precisely determined to yield accurate range estimates used to calculate the location of a mobile radio via trilateration.
In these systems, a master mobile radio transmits outbound ranging pulses to plural reference radios which respond by transmitting reply ranging pulses that indicate the location of the reference radio and the pulse turn around time (i.e., the time between reception of the outbound ranging pulse and transmission of the reply ranging pulse). Upon reception of the reply ranging pulse, the master radio determines the signal propagation time, and hence range, by subtracting the turn around time and internal processing delays from the elapsed time between transmission of the outbound ranging pulse and the time of arrival of the reply ranging pulse. In this manner, the individual radios do not need to be synchronized to a common time reference, thereby eliminating the need for highly accurate system clocks required in conventional time-synchronized systems. The brief ranging pulses can be interleaved with voice and data messages or incorporated into a messaging scheme in a non-intrusive manner to provide location detection capabilities without disruption of voice and data communications.
Time of arrival of ranging pulses must be precisely estimated. By performing internal delay calibration, errors caused by difficult-to-predict internal transmitter and receiver delay variations can be minimized. The doppler shift of each arriving ranging pulse is estimated and compensated for in determining the pulse's time of arrival. State-of-the-art spread spectrum chipping rates and bandwidths to reduce multipath interference, and yet take advantage of existing hardware and software to carry out portions of the TOA estimation process, where practical. Leading edge curve fitting is used to accurately locate the leading-edge of an acquisition sequence in the ranging pulse in order to further reduce effect of multipath interference on TOA estimates. Frequency diversity is used to orthogonalize multipath interference with respect to the direct path signal, wherein an optimal carrier frequency and phase is identified and used to estimate the TOA to minimize the impact of multipath interference.
The systems disclosed in the aforementioned patent applications are self-healing. Unlike conventional systems, which require communication with a certain set of fixed-location reference radios, the systems disclosed can use a set of reference radios that includes fixed and/or mobile radios, wherein the set of radios relied upon to determine the location of a mobile communication device can vary over time depending on transmission conditions and the location of the mobile communication device. Any combination of fixed or mobile radios of known locations can be used as the reference radios for another mobile radio in the system, thereby providing adaptability under varying conditions.
The location of the disclosed communications devices can be estimated with high levels of accuracy and reliability. Depending upon the levels of position accuracy, low probability of detection, and/or confidentiality required, communications devices of varying capability can be assembled based upon a modular architecture that leverages standard commercial hardware and communications protocols, where applicable. For example, disclosed curve fitting and frequency diversity features need only be included in a communications device if the enhanced location accuracy provided by such features is required. By such modular design, the disclosed technology is capable of supporting the most stringent of military special force requirements, as well as the reduced operational requirements demanded by civilian search and rescue operations. The ranging and location techniques presented are useful in wide variety of applications, including location and/or tracking of people and items such as: military personnel and equipment, emergency personnel and equipment, valuable items, vehicles, mobile telephones, children, prisoners and parolees.
Real world conditions will always exist in which position estimates, by even the most sophisticated location estimation system, will not be able to assure success in “physically locating” a target communications device. Factors such as long distances and obstructions between the target devices and reference devices, signal degradation, and multipath distortion will impact the accuracy of trilateration based location estimates, despite the sophistication of the system deployed. Furthermore, mission environments are often complicated by poor human operating conditions, such as darkness, smoke, heavy fog, and/or physical obstructions, resulting in situations in which even the smallest of position estimate inaccuracy can cause a search to be unsuccessful. Even highly sophisticated position estimating techniques, such as those disclosed in patent applications '702 and '625, may yield insufficient accuracy to guarantee operational success in a number of mission environments, such as those described in the following scenarios.
A skier is buried under several feet of snow, somewhere within a two acre snow field, and there are no reference communications devices inside the valley in which the avalanche occurred. Under such conditions, a conventional position estimating system, is unlikely to provide sufficient accuracy to allow rescuers to physically locate the buried individual before he suffocates. Similar difficulties arise in a scenario in which an escaped convict, who had been previously equipped with a tracking device, has taken measures to conceal himself within the “urban canyons” of a large metropolis. In such environments the exterior walls of city buildings create serious signal degradation and multipath conditions. Sophisticated, trilateration based location estimation techniques alone, in such an environment, may not provide sufficient accuracy to allow law enforcement officers to plan a safe approach by which to apprehend the convict, thereby resulting in increased risk to law enforcement officers. A final scenario requires that an electronically tagged crate stored in a warehouse, a ship's cargo hold, or other highly refractive, multipath environment, be quickly located. The inability to guarantee the ability to locate a target device in such a worst case scenario could result in the rejection of radio transponder technology that would otherwise have proven to be a highly effective tool for use in transit material tracking.
In all these scenarios, the position estimate accuracies are affected by a number of factors, including distance, signal degradation, and multipath distortion. Even if multiple position estimation communications devices were moved into the surrounding area, the initial position estimate may still not provide sufficient accuracy to assure a successful search. In many situations it is not possible to increase the number of, or to properly locate, additional communications device reference points.
Increasing the accuracy of the position estimation system may increase the cost, size, weight, and power consumption of the respective communications devices used in the 1 position estimation system, but may not assure success in physically locating a target device under all possible conditions. Furthermore, such increased accuracy would be unnecessary in support of a large number of the missions for which such location estimation systems are intended.
Accordingly, there remains a need to increase the probability of a successful physical search, despite the level of accuracy achieved by the position estimation system used to guide the search device to the general vicinity of the target device. Such a capability would allow a less accurate position estimation system to be used without sacrificing the probability of a successful search. Operationally viable position estimation systems can be assembled using communications devices of lessor and/or varying position estimation accuracy, allowing optimization of cost, size, weight, and power related considerations, while providing extremely high assurance that a designated device can be quickly and effectively located.